An apparatus includes a first oscillator circuit coupled to a first electrode and a second oscillator circuit coupled to a second electrode. The first and second oscillator circuits oscillate synchronously in response to a capacitance between the first and second electrodes being greater than or equal to a threshold coupling capacitance and asynchronously in response to the capacitance being less than the threshold coupling capacitance. The first and second electrodes are separated by a distance, such that a disturbance within the distance increases the capacitance between the electrodes equal to or above the threshold coupling capacitance. The frequency of the first oscillator circuit is inversely proportional to a capacitance of the first electrode, and the frequency of the second oscillator circuit is inversely proportional to a capacitance of the second electrode.